alpha(5)GABA(A) receptors play a pronociceptive role and avoid the rate-dependent depression of the Hoffmann reflex in diabetic neuropathic pain and reduce primary afferent excitability

摘要

Diabetic neuropathy is an incapacitating complication in diabetic patients. The cellular and molecular mechanisms involved in this pathology are poorly understood. Previous studies have suggested that the loss of spinal GABAergic inhibition participate in painful diabetic neuropathy. However, the role of extrasynaptic alpha(5) subunit-containing GABA(A) (alpha(5)GABA(A)) receptors in this process is not known. The purpose of this study was to investigate the role of alpha(5)GABA(A) receptors in diabetes-induced tactile allodynia, loss of rate-dependent depression (RDD) of the Hoffmann reflex (HR), and modulation of primary afferent excitability. Intraperitoneal administration of streptozotocin induced tactile allodynia. Intrathecal injection of alpha(5)GABA(A) receptor inverse agonist, L-655,708, produced tactile allodynia in naive rats, whereas it reduced allodynia in diabetic rats. In healthy rats, electrical stimulation of the tibial nerve at 5 Hz induced RDD of the HR, although intrathecal treatment with L-655,708 (15 nmol) abolished RDD of the HR. Streptozotocin induced the loss of RDD of the HR, while intrathecal L-655,708 (15 nmol) restored RDD of the HR. L-655,708 (15 nmol) increased tonic excitability of the primary afferents without affecting the phasic excitability produced by the primary afferent depolarization. alpha 5GABA(A) receptors were immunolocalized in superficial laminae of the dorsal horn and L4 to L6 dorsal root ganglion. Streptozotocin increased mean fluorescence intensity and percentage of neurons expressing alpha(5)GABA(A) receptors in dorsal horn and L4 to L6 dorsal root ganglia in 10-week diabetic rats. Our results suggest that spinal alpha(5)GABA(A) receptors modulate the HR, play an antinociceptive and pronociceptive role in healthy and diabetic rats, respectively, and are tonically active in primary afferents.